This invention relates to an optical information recording apparatus which operates through the projection of a focused laser beam onto a recording medium.
There have been known optical disk apparatus for recording digital data or video signals through the projection of a laser beam onto a disc-shaped recording medium. In these optical disk apparatus, recording power applied to the disk has a significant influence on the quality of record, and therefore recording with optimal power application is crucial. A conventional method for practicing this scheme is as follows, as disclosed in JP-A-59-193545 issued in 1984. In recording information signals on a recording medium through the projection of a recording beam, a signal is initially recorded by the application of a varying power. The recorded signal is reproduced thereby to find the optimal recording power which yields the best state of reproduced signal. Generally, a maximum amplitude of a signal reproduced on an optical disk provides the best signal quality, and accordingly the best state of reproduced signal implies a reproduced signal with the maximum amplitude. The above-mentioned conventional method is also designed to determine the optimal power by detecting the operation which provides the maximum amplitude (peak-to-peak value) of the reproduced signal.
However, the conventional method bases the determination of the optimal recording power on the best state of reproduced signal, and therefore it involves a problem that the determined power is not optimal for the optical disk apparatus.
The upper part (a) of FIG. 8 shows the peak power characteristics of a general optical disk, and the lower part (b) of FIG. 8 shows record marks on the recording medium produced by power with certain peak values. The term "peak power" means the above-mentioned recording power. In the upper part (a) of FIG. 8, the horizontal axis represents the peak power and the vertical axis represents the amplitude or S/N of reproduced signals, and in the lower part (b) of FIG. 8, indicated by 31, 32 and 33 are record marks produced by respective peak powers along the track direction indicated by the arrow. Peak power from 0 to P1 is the range of insufficient power, in which satisfactory record marks are not produced and reproduced signals have insufficient amplitudes. P1 is the smallest peak power to produce a satisfactory record mark. As the peak power increases from P1 to P2, record marks become larger, producing greater amplitude of reproduced signals. When the peak power exceeds P2, the record mark becomes to have duties above 50%, and the amplitude of the reproduced signal begins to fall due to a deficient resolution. When the peak power further increases beyond P3, the destruction of recording medium begins and the reproduced amplitude falls sharply. In conclusion, the peak power which provides the best state of reproduced signal is P2. The above-mentioned peak power characteristics vary depending on the type of recording medium as shown by 34, 35 and 36 in FIG. 9. The recording medium 34 has the peak power P2 for the maximum (best) reproduced signal emerging nearer to the P1, the recording medium 36 has the P2 nearer to the P3, and the recording medium 35 has the P2 amid the P1 and P3. FIG. 9 has the same coordinates as the (a) part of FIG. 8. The optimal power for the optical disk apparatus is chosen at a peak power P4 which is slightly higher than the center of the power range suitable for recording and reproduction (e.g., the range between P1 and P3 on the peak power characteristics of FIG. 9) in consideration that the emergence of some abnormality in data recording (e.g., out-tracking of the servo system due to a shock or vibration, variation of peak power due to a temperature change, presence of dust particles on the disk or lens, etc.) results in a virtual variation of peak power on the disk. The reason for the slightly higher setting of the peak power is based on the fact that the emergence of the above-mentioned abnormality results in a virtual reduction of peak power in many cases. In FIG. 9, value X is called "power margin", and it represents the admissible power reduction for signal reproduction without the occurrence of error.
On this account, the peak power which provides the best state of reproduced signal for the optical disk (recording medium), i.e., those shown by P34, P35 and P36 in FIG. 9, in not necessarily consistent with the optimal peak power P4 for the optical disk apparatus, and therefore it is difficult for the conventional optical disk apparatus, which determines the peak power based on the best recording state of reproduced signal, to find the optimal peak power for the apparatus.
Moreover, the conventional method cannot be applied to optical disk apparatus with the ability of overwriting. FIG. 10 shows the method of light projection for overwriting a phase-mode optical disk. In the figure, shown by (a) is the light modulation waveform, (b) is a recording track before overwriting, and (c) is a recording track after overwriting. Indicated by 40 is the bias power, 41 is the peak power, 42 is the crystalline state, and 43 is the amorphous state. Phase-mode material allows overwriting of signals based on its different optical reflectivity between the amorphous state and the crystalline state. Overwriting of signals is the capability of recording a new signal without erasing the old signal which has been recorded in the past. The amorphous state and crystalline state are attained by light modulation at two laser power levels, i.e., peak power and bias power, as shown in FIG. 10. Namely, regardless of the state of a recording track before overwriting, a portion where the peak power is applied becomes amorphous and a portion where the bias power is applied becomes crystalline, and this allows overwriting of a new signal. Although the optimal bias power and optimal peak power need to be set for an apparatus with the ability of overwriting, the conventional apparatus cannot set two power levels necessary for overwriting.